A rotation output device has been developed for a device that, for example, rotates an object to be processed in order to make processing thereof easier. This type of rotation output device comprises a rotating disk that fixes the object to be processed, and a reduction gear transmission reduces the speed of rotation of a motor and causes the rotating disk to rotate. An example of a rotation output device is set forth in Japanese Patent Application Publication No. H11-347868 (Patent Document 1). In the rotation output device of Patent Document 1, a reduction gear transmission is attached to an output shaft of a motor, a pinion is fixed to an output shaft of the reduction gear transmission, and the pinion meshes with internal teeth formed on an inner circumference of a rotating disk.
A reduction gear transmission that is capable of realizing a large speed reduction ratio, in which backlash is small, and in which position fixing reproducibility is high is desired. A reduction gear transmission capable of meeting this requirement is set forth in Japanese Examined Patent Application Publication No. H5-13794 (Patent Document 2). The reduction gear transmission of Patent Document 2 comprises a crankshaft, an external gear, and an internal gear. The crankshaft includes an eccentric cam that rotates eccentrically when the crankshaft rotates. The external gear comprises a hole engaging with the eccentric cam, and the external gear revolves orbitally when the eccentric cam rotates eccentrically. The internal gear has a number of teeth differing from the number of teeth of the external gear, surrounds the external gear while meshing with the external gear, and allows the orbital revolution of the external gear.
In this reduction gear transmission, the internal gear rotates when the rotation of the external gear is restricted. Conversely, the external gear revolves orbitally while rotating when the rotation of the internal gear is restricted. This type of reduction gear transmission may be called a reduction gear transmission of an inner meshing planetary gear type.